Considerable progress has been made in turbojets in terms of reducing fuel consumption and increasing specific power, by improving the thermodynamic cycle.
This improvement has been obtained in particular by increasing the pressure of the air fed to the combustion chamber, and by increasing the temperature at the inlet to the turbine.
However, increasing temperature and the inlet to the turbine requires the blades of the distributor and the blades of the first stage of the high pressure turbine that drives the high pressure compressor to be cooled energetically, since said blades are parts that are subjected to the highest temperatures and to high levels of stress. Such cooling is generally performed by bleeding a flow of air under pressure from the last stage of the high pressure compressor and passing it through the hot zones.
In modern turbojets, having a high compression ratio, the high pressures generated by the compressor, which are favorable for the thermodynamic efficiency of the engine, lead to a significant increase in the temperature of the air delivered to the combustion chamber. This increase in temperature at the outlet from the compressor requires the rate at which air is taken for cooling to be increased in order to cool the high pressure turbine, and that reduces thermodynamic efficiency.
In order to remedy that drawback, proposals have already been made to cool the bleed flow rate of air in a heat exchanger whose own cold fluid is constituted by cold air bled from the bypass flow, or by some other fluid such as fluid or oil.
U.S. Pat. No. 5,581,996 provides for bleeding air from the stream in the diffuser at the inlet to the combustion chamber. This bled-off air is cooled in a heat exchanger situated radially outside the combustion chamber, and is subsequently returned towards the inside of the engine via the blades of the distributor, thereby cooling them, and it also serves to cool the radially-inner portions of the combustion chamber, and possibly also the blades of the first stage of the turbine.
That document represents the state of the art closest to the invention since it provides for cooling the first wheel of the turbine by bleeding air from the outlet of the diffuser, which air is cooled in a heat exchanger and then, after being cooled, passes via the blades of the distributor.
Nevertheless, since air is taken in the radially-outer zone of the outlet stream from the compressor, there is a risk of the heat exchanger becoming clogged with dust blown out centrifugally. Furthermore, the cooled air passes via the blades of the distributor, and serves to cool the walls of said blades. This heats up the air and reduces the flow rate of air that can subsequently be used for cooling the blades of the turbine.